High Mode Number Vortex-Induced Vibration Field Experiments

Abstract This paper presents the initial results from VIV (Vortex-Induced Vibration) field testing of a long, flexible, model riser at high mode number. The experiments were designed to better understand the dynamic behavior of a long riser in uniform flow responding at mode numbers from 10 to 25 in cross-flow vibration. The 1.31 inch, (0.0333 m), diameter riser model was made from fiberglass line pipe, manufactured by Fiberspar Corp. Two model configurations with length of 201 feet (61.26m) and 401 feet (122.23m) were towed behind a vessel in a deep lake in upstate New York. Motion was recorded with twenty four evenly spaced internal tri-axial accelerometers. Observed reduced velocity and RMS displacement response levels are reported. Mean drag coefficients and hydrodynamic damping derived from measured data are compared to calculated values from formulas commonly used in engineering design of offshore systems. Introduction The Experiments at Lake Seneca, New York took place in the summer of 2004. They are part of a larger testing program developed by DEEPSTAR (A joint industry technology development project) aimed at improving the ability to model and mitigate VIV. While these particular tests were focused on investigating uniform flow conditions, a second set of tests designed for shear flow conditions was subsequently performed in November 2004 and will be reported on in the future. The initial motivation for this uniform flow VIV field experiment was to improve the understanding of VIV at high mode number. Most model testing has been conducted at low mode numbers (<10). As a consequence, low mode number VIV is quite well understood. There have been numerous tests conducted on very long cables as well, Reference [1]. In such experiments the dynamic behavior can be represented by cables of infinite length. Traveling waves dominate the response and standing wave modal behavior is not observed. Current drilling and production in 1000 to 3000 meters depths has brought on the need to understand VIV behavior at higher mode number, but still short of infinite length behavior. The principal objectives, of the MIT towed pipe experiments were:To gather vortex-induced vibration response data on a densely instrumented circular pipe at high mode number,To investigate mean drag coefficient (CD) at high mode number,To validate or improve drag coefficient prediction formulas,To test the efficacy of helical strakes at high mode numbers,To obtain statistics on the distribution of single-mode vs. multi-mode response of a pipe at high mode number,To determine the relative contribution to damage rate from the in-line and cross-flow VIV,To improve knowledge of damping factors. In this paper, RMS response, reduced velocities, drag coefficients and damping factors are presented. Experiment Description The Lake Seneca test facility was selected as the testing site, because it is a fully equipped field test station moored in calm, deep water. It was ideal for conducting a controlled test on a long circular pipe in uniform flow. This was accomplished by towing a vertical, composite pipe with a suspended bottom weight to produce the desired tension.